GROUNDWATER-LEVEL CHANGE AND GROUNDWATER-SURFACE EXCHANGE IN THE NORTHWESTERN MISSISSIPPI RIVER ALLUVIAL AQUIFER IMPOSED BY SEASONAL PUMPING AND CLIMATE CHANGE

Increased groundwater pumping and climate variability alter hydraulic gradients driving groundwater-surface water exchange. Hydraulic gradients determined from groundwater levels often lack temporal and spatial detail, so knowledge on the variability of groundwater-surface water exchange remains limited in many aquifers and hinders accurate estimation of water availability. In the northwest area of the Mississippi River Valley alluvial aquifer (NWAA), prior investigations of groundwater-surface water exchange offer limited long-term analysis of groundwater-level change from direct measurements. Yet over the last twenty years, significant changes have occurred on the major controls of the hydrologic system: 1) increased annual precipitation of >30 cm; 2) increased use of groundwater for irrigation; and 3) declines in groundwater levels further south in the aquifer, which may ultimately affect groundwater-surface exchange along the Black River. This study evaluated annual groundwater-level change using measurements made during February, March, or April (i.e. spring groundwater levels) over the last half-century and continuous groundwater-level measurements collected from February through July 2020. Analyses showed no long-term groundwater-level declines in the NWAA despite a near tripling of groundwater use from 0.4 GL/day in 1985 to >1.1 GL/day from 2000-2010. Continuous groundwater level measurements did reveal seasonal lowering of groundwater levels, regionally, up to 5 m. Unlike findings garnered from groundwater-level interpolations using sparse measurements or modeling approaches, this study shows the Black River as primarily a regional groundwater sink, although groundwater levels are affected by seasonal pumping. While this study lacks quantification of groundwater recharge, we postulate greater recharge has occurred over time from increased precipitation and possible inter-aquifer transfer that maintains regional groundwater flow towards Black River as groundwater pumping has increased. Importantly, the magnitude of change pumping has on seasonal groundwater-surface water exchange remains an open question. Our findings emphasize that while pumping modulates seasonal groundwater-surface water exchange in alluvial aquifers, annual to long-term changes are also sensitive to prevailing hydroclimatic conditions caused by climate change.